This invention relates to the field of crystal mounting, particularly to mounting crystals bearing complex circuitry.
Many crystal mounting methods have been devised to solve the twin problems of providing firm support during the wire bonding process while allowing the desired vibrational mode. Since in the past most crystals have required only two contacts, most of the solutions were relatively simple, typically either edge-mounted, as a beam, or center-mounted with a contact at the center of each surface.
As crystal applications became more sophisticated, as when used in filter networks, more contact points were required. As the operating frequency increases, the size of the crystal plate usually decreases. At the present, crystal applications may require as many as five non-grounded contacts on a one-fourth inch (0.6 centimeters) diameter plate and, no doubt, more will be required in the future.
High mass contacts such as solder bonds being no longer practicable, new bonding methods have been devised. At present, ultrasonic "scrubbing" is the foremost method, and involves a tool or machine which carries a spool of extremely fine wire. When the tool comes down upon a metal or metallized contact pad, an end of the wire is molecularly bonded to the pad by a combination of pressure and vibration. The tool then lifts, feeding out wire as it moves, then comes down the secoond contact pad and makes the second bond. The tool may then break off the wire bond from the wire still on the spool. It is obvious that the process requires that the crystal be supported at the bonding point in order for pressure to be applied. However, if the contact pads on the crystal were permanently supported, this would provide so much damping that the crystal would be unusable.
One solution was shown in a co-pending application Ser. No. 33,774, filed Apr. 27, 1979, and a divisional application derived from that application, Ser. No. 180,761, filed Aug. 25, 1980, and also now pending, assigned to the same assignee as is the present invention. The claimed solution was to support the crystal plate essentially on a small, very low pedestal on a substrate with ground connections made via the pedestal. Opposing pairs of active electrodes were located intermediate the center and the edge of the plate. Non-grounded electrodes were coupled by conductive strips on the crystal to pads near the crystal edge. During the bonding process, the wire bond was made first to a pad on the substrate, then the tool was lifted and brought down gently on a pad near the edge of the crystal, forcing the crystal down against the supporting substrate. The second bond was made with the crystal in this deformed but supported position. As the tool lifted, the crystal resumed planar form with no excessive damping. Since this method has the potential of stressing the crystal and the conductive areas, another mounting structure was needed which would also allow the crystal to be supported during the bonding process but free during operation.